There exists a variety of techniques for preparing sample bodies that are suitable for analysis in such methods as rheometry, X-ray fluorescence analysis, infrared spectroscopy, dissolution testing, etc. These techniques are confronted with the challenge of producing homogenous samples without air pockets or bubbles in a melting or solidification cycle. Many thermoplastic materials have high viscosity (>10 Pas) when in a molten state. Air located between the sample material particles during the melting process will be trapped as small bubbles within the melted material. It is usually the characteristics of the solid or molten sample material that are of interest in the analysis. However, many physical characteristics are strongly influenced by such embedded air bubbles. In the case of viscosity measurements, the results may e.g. vary with several decimal powers in dependency of the amount of air trapped within the sample and thereby cause inacceptable measuring uncertainty.
There are in principle two ways of controlling the amount of embedded air pockets in the sample:
(1) The sample may be melted under atmospheric conditions and the air bubbles rise due to the density difference and escape from the upper surface of the sample body. Due to the high viscosity, the speed with which the trapped air bubbles rise is very low. If the air bubbles are supposed to escape as a result of the lifting force only, a long time is needed. However, in many cases maintaining the sample at a high temperature for a long time may on the other hand lead to non-desirable sample degeneration and may thus affect the sample and the analysis result.
(2) The air bubbles may be prevented by melting the sample in a vacuum. The starting materials are evacuated at room temperature in a hermetically sealed chamber and then melted. No air is present between the particles during the melting, such that the particles melt into a homogenous sample. The critical influencing factors of this variation are the speed of evacuation of the sample chamber, the obtainable heating/cooling rates as well as possible sublimation or evaporation of the sample.
A variety of implementations of the above-mentioned principles exist, which are used in various situations:
A first example makes use of a vacuum oven. The sample is melted within a chamber that can be evacuated heated. The starting materials, i.e. powder or pellets, are placed in a mould or on a non-stick foil. Long cycle durations are usually needed, since such ovens due to their comparatively large volumes only allow for slow temperature control and evacuation. A further drawback of this method is the usually non-defined shape of the sample body. A clear advantage of this variant is that such an oven is commonly available as part of standard laboratory equipment.
A second example relies on a platen press, which consists of a movable plate and a stationary plate. Both plates are usually heatable and a well defined force may be applied to the movable plate, e.g. by means of hydraulics. Due to their versatility, platen presses belong to standard laboratory equipment. In such a press, the sample bodies are produced in special planar moulds. The mould is separated from the base plate by a separation foil and pellets or powder is filled into the openings of the mould and melted. The movable plate is pressed onto the mould and the samples are formed under well defined conditions. The drawbacks of this method comprise the facts that an expensive machine (vacuum platen press) is needed, and that the sample chamber is usually large such that it reacts slowly on changes in temperature and pressure. Samples produced in accordance with this method may further comprise shrinking cavities. This is due to the fact that the sample chamber volume is defined by the mould and may not adapt to the various conditions during production. The sample chamber is initially filled with the starting materials. Since solid bulk material has lower density than the pure substance, the mould is overfilled and a volume contraction occurs during the melting which often causes cavities to occur within the sample. Excess material due to overfilling is pressed between separation foil and mould.
A third example is based on a pellet pressing machine which is similar to machines used for pressing pills or tablets. In these machines, a chamber is filled with the starting materials (pellets or powder) and compressed from above by a punch. The pressing force is provided by means of a hydraulic or mechanical press. The compacting may take place under increased temperature and vacuum. Thereafter, the sample body is removed from the pressing tool. This step is problematic in connection with heated pressing, since many materials adhere to the surfaces of the pressing tool and therefore can only be released by application of extensive force. Such application of force often leads to destruction of the sample body. Therefore, this approach is only applicable to substances which only adhere weakly to the pressing tool or which can resist the mechanical loads during removal.
A fourth example uses an injection molding machine. In this case, the material is melted within a chamber and injected through a nozzle into a cold separable mould. This method provides sample bodies with homogenous shape and mass, when the melting takes place under vacuum. Accordingly, this way of sample body preparation constitutes the benchmark method. However, the main drawback is the significant cost with regards to acquisition of the necessary equipment as well as process development.
There may thus be a need for a simple and effective way of preparing homogenous sample bodies from thermoplastic material. In particular, there may be a need for a way of preparing such sample bodies in a rapid manner and at a relatively low cost.